In radio astronomy, the flux unit or Jansky (symbol Jy) is a non-SI unit of electromagnetic flux equivalent to 10−26 Watts per square meter per Hertz. The unit "Jansky" is named after the pioneering radio astronomer Karl Jansky.

The brightest natural radio sources have flux densities of the order of one (to one hundred) Jansky, which makes the Jansky a suitable unit for radio astronomy.

For example the Third Cambridge Catalogue (3C) which was prepared in 1959 and revised in 1962, lists some 300 to 400 radio sources in the Northern Hemisphere brighter than 9 Jy at 159 MHz.

1 Jy = 10-26 W m-2 Hz-1 (SI)

1 Jy = 10-23 erg s-1 cm-2 Hz-1 (cgs)

The flux density in Jy can be converted to a Magnitude basis, for suitable assumptions about the spectrum. For instance, converting an AB magnitude to a flux-density in microjansky is straightforward:[1]

Fv [µJy] = 1029 10-(AB+48.6)/2.5


  1. M. Fukugita (1995). "Galaxy Colors in Various Photometric Band Systems". PASP 107: 945-958.